/Engine/Source/Runtime/AudioMixer/Public/AudioMixerSubmix.h
Calls ProcessAudio on the child submix, performs all necessary conversions and mixes in it's resulting audio.
void MixInChildSubmix ( [FChildSubmixInfo](API\Runtime\AudioMixer\FChildSubmixInfo) & Child, [ISoundfieldAudioPacket](API\Runtime\AudioExtensions\ISoundfieldAudioPacket) & PacketToSumTo )